Wipe ptls->system_id when a thread exits.

On macOS, we use the system thread ID to match against the list of known
thread local states during signal handling. To prevent picking up the
wrong entry, i.e. from when a thread was previously executing a
different task, make sure to wipe the system ID when a thread exits.

This manifested as the signal handler actually reporting a bus error
when a thread touched safepoint memory during GC, because the matched
thread local state had no current task attached to it.

Fixes JuliaGPU/Metal.jl#225

src/threading.c

@@ -474,6 +474,7 @@ static void jl_delete_thread(void *value) JL_NOTSAFEPOINT_ENTER
         abort();
     }
     jl_atomic_store_relaxed(&ptls->current_task, NULL); // dead
+    ptls->system_id = 0;
     // finally, release all of the locks we had grabbed
 #ifdef _OS_WINDOWS_
     jl_unlock_profile_wr();